THERMODYNAMIC OPTIMISATION AND EXPERIMENTAL … · 2015-02-03 · 1 THERMODYNAMIC OPTIMISATION AND...

1

THERMODYNAMIC OPTIMISATION AND

EXPERIMENTAL COLLECTOR OF A DISH-

MOUNTED SMALL-SCALE SOLAR

THERMAL BRAYTON CYCLE

Department of Mechanical and Aeronautical Engineering,

University of Pretoria, South Africa

February, 2015

WG LE ROUX

Study-leaders:

Prof. T. Bello-Ochende

Prof. J.P. Meyer

Submitted in partial fulfilment of the requirements for the degree PhD (Mechanical Engineering)

2

Presentation Outline

1. Introduction

2. Background

3. Literature Study

4. Modelling and Optimisation

5. Analytical Results

6. Experimental Study

7. Conclusion

8. Recommendations

3

1. Introduction

Long-term average of direct normal solar irradiance

on a world map showing the potential of solar power generation in

southern Africa (GeoModel Solar, 2014)

Parabolic dish concentrator for a Stirling engine (Image extracted from Pitz-Paal, 2007)

A typical micro-turbine (the

GT1241) as available from Honeywell,

Garrett proposed for the small-scale solar

thermal Brayton cycle (Image extracted from Garrett, 2014)

4

1. Introduction

Problem • Solar-to-electricity technologies are required which are

• more competitive

• more efficient

• cost-effective

Purpose of the study Small-scale dish-mounted open solar thermal Brayton cycle

• optimise solar receiver and recuperator - method of total entropy generation minimisation

• test optimised receiver

Objectives • Second law of thermodynamics

• Entropy generation minimisation

• Ray-tracing software

• Geometry optimisation

• Experimental receiver test

Scope of Research – Thermodynamic Optimisation

• Open and direct solar thermal Brayton cycle

• Second Law of Thermodynamics

• Entropy Generation Minimisation

• Maximise net power output

• Optimise geometry of recuperator and receiver

• Heat Transfer & Fluid Flow Irreversibilities

• Experimental setup

2. Background

Solar resource – South Africa

Why Solar?

Solar resource - World

• According to DLR

Solar resource – South Africa

Why Solar?

The Department of Minerals and Energy places South Africa’s

annual direct normal irradiation (DNI) between 2 500kWh/m2

and 2 900 kWh/m2 with an average of almost 300 days of

sunshine per year.

0

200

400

600

800

1000

1200

1400

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Irra

dia

nce

(W/m

^2)

Time (h)

Irradiance of beam

Mean irradiance of globalradiation, tracked

Mean irradiance of globalradiation horizontal

Solar resource – South Africa, Pretoria

Meteonorm

CSP - Concentrating methods

Dish Trough Tower

12

Background

Brayton cycle • mobility, cost benefits

• micro-turbines

• hybrid system

• storage systems

• water heating

• efficient and highly competitive.

Maximum net power output • combined effort of

• heat transfer,

• fluid mechanics and

• thermodynamics

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

Small-scale solar power

• Photovoltaic cells

• Solar water heaters

• CSP (Concentrated solar power)

– Trough

• Rankine Cycle

– Dish-mounted

• Stirling Engine

• Brayton cycle

Solar tracking - Elevation

• SunEarthtools

Solar tracking - Azimuth

-150

-100

-50

0

50

100

150

6 7 8 9 10 11 12 13 14 15 16 17 18

Az

imu

th a

ng

le

Time (h)

Morning measurements

Noon measurements

Afternoon measurements

SunEarthTools

Measured angle of tracking

system versus real azimuth angle

of the sun

Mousazadeh et al. (2004), Poulek and Libra (2000)

Two-axis solar tracking required for dish Solar

tracking Active

Passive Micro-

processor

and

electro-

optical

sensor

based

Auxiliary

bifacial

solar cell

based

Date and time

based or a

combination

of sensor and

date/time

based

Fluid Bi-

metallic

strips

16

3. Literature Study

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

The open and direct solar thermal Brayton cycle

17

3. Literature Study

Test set-up of a solar thermal Brayton cycle (Image extracted from Heller et al., 2006)

Small-scale open and direct solar thermal

Brayton cycle with recuperator • Advantages

– High recommendation – Air as working fluid – Hot air exhaust

• Water heating • Space heating • Absorpsion refrigeration

– Recuperator • high efficiency and • low compressor pressure ratios

• Disadvantages

– recuperator and receiver pressure losses – turbo-machine efficiencies – recuperator effectiveness – Heat losses

irreversibilities

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

Solar thermal Brayton - Recuperator

Solar thermal Brayton - Recuperator

Image extracted from: Stine, B.S., Harrigan, R.W., 1985, Solar energy

fundamentals and design. New York: John Wiley & Sons, Inc.

21

3. Literature Study

Receiver

type

Reference

number or

model

Tout (K) Tin (K) P (kPa) (kg/s) Working fluid ΔP (Pa)

Pressurised

volumetric

PLVCR-5

(Ávila-Marín,

2011)

71% 1 323 - 420 - Air -

PLVCR-500

(Ávila-Marín,

2011)

57% 1 233 300 415 - Air -

DIAPR

(Karni et al.,

1997),

(Ávila-Marín,

2011)

79% 1 477 308 1 800 0.0222 Air 25 000

REFOS

(Buck et al.

2002),

(Ávila-Marín,

2011)

67% 1 073 - 1 500 - Air 1 800

Dickey, 2011 88% 871 542 273 0.409 Air 2 900

recm

Efficiencies of different solar receivers – Pressurised volumetric

22

3. Literature Study

Efficiencies of different solar receivers - Tubular

Receiver

type

Reference

number or

model

Tout (K) Tin (K) P (kPa) (kg/s) Working fluid ΔP (Pa)

Tubular Cameron et

al., 1972

51%* 1 089 865 370 0.73 He-Xe 7 000

Kribus et al.,

1999

- 1 023 300 1 600 -

1 900

0.01 Air 40 000

Heller et al.,

2006

- 823 573 650 - Air 10 000

Neber and

Lee, 2012

82% 1 500** - 760 0.0093 Air 40

Amsbeck et

al., 2010

43% 1 076 876 384 0.526 Air 7 330

Amsbeck et

al., 2010

39.7% 1 055 871 375 0.516 Air 7 400

Solugas

(Quero et

al., 2013)

- 873 598 850 5.6 Air

*calculated by author

**proposed

rec m

23

3. Literature Study

Particle receiver (Image extracted from Miller and Koenigsdorff,

1991)

Open volumetric receiver – HiTRec (Image extracted from Ávila-Marín, 2011)

24

3. Literature Study

Closed volumetric receiver,

REFOS (Image extracted from Buck et al., 2002)

Longitudinal tubular receiver (Image extracted from Amsbeck et al., 2008)

25

3. Literature Study

Coiled tubular receiver (Image extracted from Kribus et al., 1999)

Ceramic counterflow plate-type

recuperator (Image extracted from Pietsch and Brandes, 1989)

26

3. Literature Study

0

2

4

6

8

10

12

1.4 1.6 1.8 2 2.2 2.4

(kW

)

Q1 Q2 Q3

Q4

Q5

Q6 Q7

T1

T2

T3

T4

Q1 = 6.8 kW, T1 = 1 308 K, Q2 = 8.3 kW, T2 = 1 179 K, Q3 = 9.7 kW, T3 = 1 054 K, Q4 = 11.2 kW, T4 = 904 K Q5 = 12.7 kW, Q6 = 14.1 kW, Q7 = 15.9 kW

Performance map

(in different weather conditions)

• small-scale open solar thermal

Brayton cycle

• fixed optimised geometries

27


ctnet WWW

*Q

j

jlossQ ,

m

m

Control volume for the open solar thermal Brayton cycle

28


Example of an analysis done for the solar dish and receiver

Solar receiver - SolTrace

29


Rectangular open-cavity

solar receiver

Heat loss from the

open-cavity receiver

Solar receiver

30

4. Modelling Solar receiver air heating

• Rectangular open cavity tubular receiver

• Stainless steel

• Pressure drop (Colebrook equation)

Variables

• Tube diameter,

• Inlet temperature,

• Mass flow rate

31

4. Modelling Solar receiver –

conduction heat loss [1]

Assumptions:

• Wind speed: 2.5 m/s

• T0 = 300 K

• P0 = 86.6 kPa

• 100 mm insulation thickness

• Conductivity of 0.061 W/mK at 550 °C average temperature [2]

• Elevation angle of 45 °

ninsinsnout

ns

cond

nsn

ncondlossAktAh

TT

R

TTAQ

//1

,,

,,

77.1)//1( insinsout kth

[1] Le Roux, W.G., Bello-Ochende, T. and Meyer, J.P., 2014, The efficiency of an open cavity

solar receiver for a small-scale solar thermal Brayton cycle, Energy Conversion and

Management 2014;84:457–70.

[2] Harris, J.A., Lenz, T.G., 1983, Thermal performance of solar concentrator/cavity receiver

systems, Solar Energy 34 (2), pp. 135-142.

32


radiation heat loss

44

,,, TTAQ nsapradnloss

N

j

jsjnsnjnnn TTFAQ1

4

,

4

,

33


convection heat loss [2]

[1] Le Roux, W.G., Bello-Ochende, T. and Meyer, J.P., 2014, The efficiency of an open cavity

solar receiver for a small-scale solar thermal Brayton cycle, Energy Conversion and

Management 2014;84:457–70.

[2] Harris, J.A., Lenz, T.G., 1983, Thermal performance of solar concentrator/cavity receiver

systems, Solar Energy 34 (2), pp. 135-142.

TTAwhQ nsninnernconvloss ,,,

4/12.3

2

Pr)(cos52.0

2

L

innercav

Gr

ahNu

For aopt = 0.25 m [1]:

hinner = 2.75 W/m2K

w = 2

Koenig and Marvin heat loss model [2]

34


Recuperator geometry

Recuperator design in SolidWorks

35

4. Modelling Recuperator

Lreg

a

b

t H

• Counterflow plate-type recuperator

• Pressure drop : Colebrook equation

• Fully developed laminar flow

• t = 1 mm

• Geometry variables: a, b, L, n

36

4. Modelling Recuperator

Efficiency modelling:

Updated version of the ε-NTU – method [3]

• Includes heat loss to the environment

• Since recuperator operates at high temperature

1,1

1,1

1

1

hXh

hX

hCrCr

Cr

1,1

1,1

0

0

hX

h

h

X

c

Cr

CrCr

h

E

h

E

chh

X

CreCr

eCrB

11

10

110

1

h

XhchX

CrNTU

11

h

h

hhch Cr

Cr

CrNTUB

1 hh CrNTUE

[3] Nellis, G.F. and Pfotenhauer, J.M., 2005, Effectiveness-NTU relationship for a

counterflow heat exchanger subjected to an external heat transfer, Journal of Heat

Transfer 127, pp. 1071 – 1073.

cpc

hph

hcm

cmCr

,0

,0

hph

hcm

UANTU

,0

incinh

hloss

hTTUA

Q

,,

,

incinh

closs

cTTUA

Q

,,

,

37

4. Modelling Micro-turbine

Standard off-the-shelf micro-turbines from

Honeywell

• Geometry not optimised

• Compressor map

• Isentropic efficiency

• Corrected mass flow rate

• Pressure ratio

• Rotational speed

38

4. Modelling Micro-turbine

Standard off-the-shelf micro-

turbine from Honeywell

• Parameter: turbine

operating point

• Turbine map

• Corrected mass flow

rate

• Pressure ratio

• Maximum efficiency

• Efficiency as function of

pressure ratio found using

blade speed ratio (BSR)

2/11

12

260

2

k

k

tin

t

rh

DN

BSR

2

max,6.0

6.01

BSRtt

39

4. Modelling Receiver heat flux

• Receiver heat flux determined with SolTrace • Solar tracking error of 1° • Optical error of 10 mrad

• Dish reflectivity of 85%

• Direct normal irradiance of 1 000 W/m2

40

4. Modelling Net absorbed heat rate

Determined for each tube section

0

0,

1

1 0

,

,

,

2

11

pn

in

n

i p

inet

ns

nnet

cmhA

Tcm

QT

Q

TTR

AcTmA

TFAcTmFA

cTmAQQ

RTTATTAh

TTFA

TTFAQQ

QQQQQ

ns

cond

n

nsn

nnn

N

j

jsjjnn

nsnnnsolarnnet

condnsnnsnn

jnsnnn

N

j

jsjnsnjnnnsolarnnet

ncondlossnconvlossnradlossnsolarnnet

,2,2

4

1

1,1

1,1,,

,,

44

,

1

4

,

4

,,,

,,,,,,,,

/

• Equations are solved

simultaneously with

Gaussian elimination

• Radiation heat loss term

is linearised

41

4. Modelling Net absorbed heat rate

Determined for each tube section

• Equations are solved

simultaneously with

Gaussian elimination

• Radiation heat loss term

is linearised

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25Tube position - bottom to top

42

4. Modelling

Net power output

11

101110int, ln*

*1

T

TcTmTTcmQ

T

TSTW ppgennet

8989890

87870

6767670

56560

4545450

/

39

410

39

410

0

2323230

21210int,

/ln/ln/

/ln/ln

/ln/ln/

/ln/ln*

*

/ln/ln/

/ln

/ln/ln/

/ln/ln

0

Ductpl

turbinep

Ductpl

receiver

p

loss

Ductpl

rrecuperato

l

cR

p

Ductpl

compressorpgen

PPRmTTcmTQ

PPRmTTcm

PPRmTTcmTQ

PPRmTTcmT

Q

T

Q

PPRmTTcmTQ

TQPP

PP

TT

TTcm

PPRmTTcmTQ

PPRmTTcmS

p

519/460

7.14/

7

7

T

Pmm tCF

t

• Steady-state

temperatures and

pressures found with

iteration, written in

terms of isentropic

efficiencies,

recuperator efficiency,

geometry variables

Objective function:

43

4. Modelling

Net power output

Assumptions:

• Connecting tubes

• Insulation

• 0.18 W/mK conductivity

• 10 mm thick

• T8 = T9, T2 = T3

• P8 = P9, P2 = P3

• V1 = V11

• Z1 = Z11

• Pressure drop – Colebrook equation (rough stainless steel friction factor)

• T1 = 300 K

• P1 = P10 = P11 = 86 kPa

• Steady-state temperatures and pressures found with iteration, using

isentropic efficiencies, recuperator efficiency

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

44

4. Modelling

Net power output

MATLAB:

For 3 different receiver tube diameters

For 5 different micro-turbines

For the different operating points of the turbine

For 625 different recuperator geometries

Find temperatures and pressures in the cycle with iteration

Determine net power output

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

Optimisation:

Run through all different

combinations of receiver

diameters, recuperator

geometries, micro-

turbines and micro-turbine

operating points

45

4. Modelling

Constraints

• Maximum receiver surface temperature

• 1200 K

• Recuperator total plate mass • 300 kg

• 400 kg

• 500 kg

46

4. Modelling - Flownex

Flownex modelling of the small-scale solar thermal Brayton cycle.

47


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.01 0.02 0.03 0.04 0.05 0.06

Optical error = 5 mrad




Optical efficiency of a solar dish and receiver with a tracking error of 1°

• SolTrace

A’ =Area ratio (Aaperture/Aconcentrator)

48


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.002 0.004 0.006 0.008 0.01






Overall receiver efficiency for a solar tracking

error of 1° with receiver surface emissivity of 0.7

• Heat loss

A’ =Area ratio (Aaperture/Aconcentrator)

49


0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10

Hea

t fl

ux

(kW

/m^2

)

Position - bottom to top

Top

Side1

Opposite Side1

Side2

Opposite Side 2

Heat flux rate at different positions on the different receiver inner walls for a tracking error of 1° • SolTrace

50


0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25Tube position - bottom to top

Temperatures and net heat transfer rates for a 0.0833 m receiver tube

diameter with a tracking error of 1° and optical error of 10 mrad.

51


0

0.5

1

1.5

2

2.5

1.2 1.4 1.6 1.8 2 2.2

Wn

et

(kW

)

rt

D = 0.0833

D = 0.0625

D = 0.05

Maximum net power output of the solar thermal Brayton

cycle with a micro-turbine selected from Garrett

rt = Turbine pressure ratio

D = Receiver tube diameter (m)

52


0

200

400

600

800

1000

1200

1 2 3 4 5 6 7 8 9 10

T (K

)

Position in the cycle

Flownex

Matlab

Predicted temperatures at different positions in the solar thermal Brayton cycle

• Matlab model

• Flownex model

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW

Receiver

11

5

*Q

2

Turbine

Micro-turbine GT2560R at 87 000 rpm

53


54

6. Experimental Study Solar dish and

tracking system

Assembly of 4.8 m diameter parabolic solar dish in the laboratory (upside down):

Test set-up showing solar dish on two-axis solar tracking system:

• SolidWorks

• As constructed

for experiment

55

6. Experimental Study Solar dish and

tracking system

56


-20

-15

-10

-5

0

5

10

1 2 3 4 5 6 7 8 9 10 11 12

Err

or

(mm

)

Segment number

Pre-assembly

On tracker

Measured error of the end-height of the

12 dish arms during pre-assembly and

on the tracker:

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10 11 12

Slo

pe

err

or

(mra

d)

Segment number

Absolute slope error per dish arm as

installed on the solar tracking

system:

57


Measurement of the solar resource

Solar measuring station to measure the DNI of the sun (SOLYS 2):

• Roof of Engineering Building 1

58


Solar receiver

Manufacturing of solar receiver

59


Solar receiver

Manufacturing of solar receiver

Inlet

Outlet

Side view of solar receiver

Position of three weldpad themocouples

60


Solar receiver

On the insulation before installation

Top view of the solar receiver with

aperture shown at the bottom.

61


1

2

4

5

3

Layout of the experimental set-up.

1– Solar receiver with insulation; 2 – Leaf blower at receiver inlet; 3 – Receiver

support structure; 4 – Parabolic dish; 5 – Thermocouple wires to data logger.

Test A – With blower

Test B – Without blower

62


A bottom view of the solar receiver and its support structure

63

6. Experimental study - results

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200 1400

Tem

pe

ratu

re (

°C)

Time (s)

Top

Bottom

Air out

Air in

Test A - Receiver surface temperature and air temperature

measurements at the inlet (bottom) and outlet (top)

64

6. Experimental study

Day 1 1 2 2 2 3 3 3 3

Blower setting 6 1 5 4 3 2 4 4 3

Start time 12:37 14:36 11:18 12:27 14:26 10:13 11:24 12:25 14:28

Steady-state time 13:00 14:56 11:34 12:52 14:40 10:41 12:01 12:45 14:41

Receiver inlet (°C) 39.2 38.8 35.5 38.4 35.9 35.5 - 38.0 36.2

Receiver middle (°C) 45.5 44.9 41.7 45.7 44.4 44.6 - 46.6 45.0

Receiver outlet (°C) 50.4 50.6 46 54.1 50.0 50.1 - 52.2 48.0

Air ambient (°C) 19.8 20.4 17 16.4 18.6 15.9 18.4 19.1 19.9

Air outlet (°C) 52 51 42 49 49 46 50 52 45.0

Collector efficiency

(%)

29.5 23.2 19.9 24.9 22.4 21.0 25.3 26.3 21.2

Optical efficiency (%) 53.6 42.2 36.2 45.3 40.7 38.2 46.0 47.8 38.5

Test A - Steady-state receiver surface temperature and air temperature

measurements at the inlet, outlet and in the middle of the receiver

65


Test A - Expected ray performance of the experimental collector

during the second test of Day 2, according to SolTrace.

For a dish with

• 5 mrad slope error,

• 25 mrad specularity error,

• 1° tracking error,

• 55% dish reflectivity,

• DNI of 700 W/m2 and

• 85% receiver tube

absorptivity.

According to SolTrace, such a

collector would have an

efficiency of 21%.

This efficiency compares well

with the efficiency of 23.2%

obtained experimentally during

the second test on Day 1 when

the DNI was 700 W/m2.

66


Test B - Receiver surface temperature increase as a function of time

• No blower

273

323

373

423

473

523

573

623

673

0 5000 10000 15000 20000

Tem

pe

ratu

re (

K)

Time (s)

Top

Middle

Bottom

Insulation

Air

67


273

323

373

423

473

523

573

623

0 5000 10000 15000 20000

Tem

pe

ratu

re (

K)

Time (s)

Measured

Calculated

Test B - Receiver average surface temperature as a function of time

• as measured experimentally

• as calculated with

• h = 6.5 W/ m2K before steady state

• and h = 1 W/m2K after steady state

68


Test B - Conduction heat loss from the receiver

• as measured experimentally

• as calculated with

• h = 6.5 W/ m2K before steady state

• and h = 1 W/m2K after steady state

0

50

100

150

200

250

300

0 5000 10000 15000 20000

He

at lo

ss (

W)

Time (s)

Measured

Calculated

69


Test B - Receiver insulation change

Heat loss from the receiver at an average temperature of 590 K with

different insulation arrangements

0

10

20

30

40

50

60

70

80

90

100

1 2

Pe

rce

nta

ge o

f to

tal h

eat

loss

(%

)

Test Number

Conduction

Convection

Radiation

1

2

70


0

100

200

300

400

500

600

700

0 2000 4000 6000 8000 10000 12000

Tem

pe

ratu

re (

K)

Time (s)

Top

Middle

Bottom

Receiver insulation change

Receiver surface temperature rise after

insulation change

71

7. Conclusion

• The method of total entropy generation minimisation was found to be a holistic

optimisation approach whereby the components of the small-scale solar

thermal Brayton cycle could be optimised.

• A method to determine the surface temperatures and net heat transfer rates

along the length of the open-cavity receiver tube was presented.

• The factors contributing to the temperature and net heat transfer rate profiles

on the receiver tube were divided into two components:

• geometry-dependent and

• temperature-dependent.

• It was found that many errors existed due to the solar collector – modelled with

SolTrace

• An optimum receiver-to-concentrator-area ratio of A’ ≈ 0.0035

• for 1° solar tracking error,

• 10 mrad optical error and

• 45° rim angle was found for the open-cavity tubular solar receiver.

72

7. Conclusion

• The open-cavity tubular solar receiver surface temperature and net heat

transfer rate for heating air depended on

• the receiver size,

• mass flow rate through the receiver,

• receiver tube diameter,

• receiver inlet temperature and

• dish errors.

• Receiver efficiencies of between 43% and 70% were found for the open-

cavity tubular receiver

• with a = 0.25 m,

• 0.06 kg/s ≤ mass flow rate ≤ 0.08 kg/s,

• 0.05 m ≤ d ≤ 0.0833 m and

• 900 K ≤ Tin,0 ≤ 1 070 K,

• operating on a 4.8 m diameter dish with 10 mrad optical error and

maximum solar tracking error of 1°.

73

7. Conclusion

• The higher the mass flow rate through the receiver, the lower the surface

temperatures and the more efficient the receiver.

• A high receiver efficiency was not necessarily beneficial for the small-

scale solar thermal Brayton cycle as a whole but the second law

efficiency was more important.

• The small-scale open solar thermal Brayton cycle could generate a

positive net power output with solar-to-mechanical efficiencies in the

range of 10-20% with much room for improvement.

• Optimum receiver and recuperator geometries were found.

• Good comparison between the Matlab results and Flownex results were

found (within 8%), except for the recuperator outlet temperature, which

differed because of the use of different ε-NTU methods to calculate the

recuperator efficiency.

74

7. Conclusion

• A 4.8 m parabolic aluminium dish with rim angle of 45° and two-axis

tracking system was designed and built.

• A tubular stainless steel solar cavity receiver was built and tested.

• The efficiency of the collector was determined with a flow test.

• A high-temperature test was performed to validate heat loss models.

• The higher the inlet temperature, the less efficient the receiver

became and the higher the maximum receiver surface temperature.

• The convection heat transfer coefficient was determined

• The heat loss rate due to convection and conduction was

significantly reduced with the proper insulation arrangement.

• The use of SolTrace was validated to a certain extent.

• It is concluded that the small-scale dish-mounted open solar

thermal Brayton cycle with tubular receiver and recuperator does

have merit and it is recommended that it be investigated further

experimentally.

75

8. Recommendations

• To make the small-scale open solar thermal Brayton cycle a success:

• large receiver tube diameter,

• very precise solar tracking system,

• high-specularity, high-reflectivity dish,

• 1° tracking error and 10 mrad optical error with reflectivity

above 90% should be sufficient

• Future work

• A smaller, more accurate and efficient dish and tracking system

• Testing of the optimised open-cavity tubular receiver at a temperature of 1 150 K for fatigue

loadings and thermal expansion

• The optimised receiver should be coupled to an optimised recuperator and micro-turbine to

determine the net power output of the system experimentally

• A cost-effective high-temperature and low-emissivity stainless steel receiver coating should be

developed.

• Optimisation of the cycle at receiver surface temperatures below 700 °C so that black

chromium can be used as low-emissivity coating.

• A moulded receiver cover to insulate the receiver

• so that air cannot flow around the receiver tubes but only on the inner side of the receiver cavity

• good thermal contact between the insulation and the receiver should be achieved regardless of

thermal expansion

• thermal expansion of the receiver should be considered

76

Acknowledgements Assistance while building the fairly large experimental set-up:

• Chris Govinder,

• Donald Keetse,

• Evan Huisamen,

• Rupert Stander,

• Koos Mthombeni,

• Clyde Engineering,

• Marcelino Benjamin,

• Matsemela Zacharia (Zakes)

• Mogashoa, Milton Griffiths,

• Otto Scheffler,

• Ruan Fondse,

• Wian van den Bergh,

• Johannes Joubert,

• Andries Tiggelman,

• Bera Chirwa,

• Ryan Capitani,

• Suzanne Roberts,

• Jacob Masingi,

• Milga Manufacturing,

• Werner Scholtz,

• Phenyo Zobane,

• Erick Putter,

• Edwyn Mothabine,

• Alan Naidoo,

• Tebogo Mashego,

• Johan Clarke,

• Modupe Matolo,

• Israel Mabuda,

• Thato Mahlatji,

• James Gerber

• Zimase Dlamini.

• Prof Bello-Ochende

• Prof Meyer

• I thank my wife and my family for their

support.

This work is based on the research supported

by the National Research Foundation (NRF),

University of Pretoria, CRSES, the Solar

Hub between the University of Pretoria and

Stellenbosch University, TESP, NAC,

EEDSH Hub, Energy-IRT and the CSIR. The

financial assistance of the National Research

Foundation (NRF) towards this research is

hereby acknowledged. Opinions expressed

and conclusions arrived at are those of the

author and are not necessarily to be

attributed to the NRF.

77

I thank God for good health and an injury-free research period.

78

Journal Publications

1. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2011. Operating conditions of an open and

direct solar thermal Brayton cycle with optimised cavity receiver and recuperator. Energy, Vol.

36, pp. 6027-6036.

2. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2012. Thermodynamic optimisation of the

integrated design of a small-scale solar thermal Brayton cycle. International Journal of Energy

Research, Vol. 36, pp. 1088-1104.

3. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2012. Optimum performance of the small-

scale open and direct solar thermal Brayton cycle at various environmental conditions and

constraints. Energy, Vol. 46, pp. 42-50.

4. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2013. A review on the thermodynamic

optimisation and modelling of the solar thermal Brayton cycle. Renewable and Sustainable

Energy Reviews, Vol. 28, pp. 677-690.

5. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2014. The efficiency of an open-cavity solar

receiver for a small-scale solar thermal Brayton cycle. Energy Conversion and Management,

Vol. 84, pp. 457-470.

79

Conference papers

1. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2011. Optimum performance of the small-scale open and direct solar thermal Brayton cycle at

various environmental conditions and constraints. In: Proceedings of the International Green Energy Conference (IGEC-VI), 5-9 June, Eskisehir,

Turkey.

2. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2011. Optimum operating conditions of the small-scale open and direct solar thermal Brayton

cycle at various steady-state conditions. In: Proceedings of the 8th International Conference on Heat Transfer, Fluid Mechanics and

Thermodynamics (HEFAT2011), 11-13 July, Mauritius.

3. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2011. Maximum net power output of the recuperative open and direct solar thermal Brayton

cycle. In: Proceedings of the 5th International Conference on Energy Sustainability (ASME, ES 2011), 7-10 August, Washington, USA.

4. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2012. Optimum small-scale open and direct solar thermal Brayton cycle for Pretoria, South

Africa. In: Proceedings of the 1st Southern African Solar Energy Conference (SASEC 2012), 21-23 May, Stellenbosch, South Africa.

5. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2012. Optimum small-scale open and direct solar thermal Brayton cycle for Pretoria, South

Africa. In: Proceedings of the 6th International Conference on Energy Sustainability (ASME, ES 2012-91135), 23-26 July, San Diego, California,

USA.

6. Le Roux, W.G., Mwesigye, A., Bello-Ochende, T., Meyer, J.P., 2014. Tracker and collector for an experimental setup of a small-scale solar

thermal Brayton cycle. In: Proceedings of the 2nd Southern African Solar Energy Conference (SASEC 2014), 27-29 January, Port Elizabeth, South

Africa.

7. Le Roux, W.G., Bello-Ochende, T., Meyer, J.P., 2014. Optimisation of an open rectangular cavity receiver and recuperator used in a small-scale

solar thermal Brayton cycle with thermal losses. In: Proceedings of the 10th International Conference on Heat Transfer, Fluid Mechanics and

Thermodynamics (HEFAT2014), 14-16 July 2014, Orlando, Florida, USA.

8. Le Roux, W.G., Meyer, J.P., Bello-Ochende, T., 2015. Experimental testing of a tubular cavity receiver for a small-scale solar thermal Brayton

cycle (SASEC 2015), 11-13 May, Skukuza, Kruger National Park, South Africa.

80

Thank you

Questions?

[email protected]

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